This project involves longitudinal study of rhesus monkey biobehavioral ontogeny, emphasizing investigation of individual differences in reactions to mild environmental challenges and determination of long-term developmental consequences under standardized rearing conditions. This past year 2 new studies were initiated, data collection was extended in 3 ongoing long-term studies, and major data analyses were completed in 2 other experiments, all of which is summarized as follows: (1) A neonatal assessment system, characterizing rhesus monkey newborn reflex development and temperamental stability, was validated on 4 independent infant samples and subsequently used to identify newborns at high risk for displaying extreme reactions to future environmental challenges. (2) neonates identified as potential future high reactors (and low reactive control neonates) were cross-fostered with adult females who themselves differed in temperamental qualities and maternal rearing styles. Reactivity differences among infants were largely masked in the presence of their foster-mothers but reappeared when they were challenged via physical separation from these adult females. (3) Longitudinal study of older infants reared either by mothers or in all-peer groups revealed major rearing condition differences in behavioral reaction to separation challenge but similar patterns of physiological response, as assessed by levels of plasma cortisol and CSF monoamine metabolites. (4) Differences in social dominance among peer-reared juveniles were related to HPA reactivity differences, both of which were found to be associated with paternal pedigree. (5) High and low reactive juvenile and adolescent monkeys were compared in their biobehavioral response to the antidepressant imipramine, while (6) a new study examining their performance on cognitive tests of short-term memory under both baseline and challenge conditions was initiated. Finally, (7) direct comparisons with rhesus monkeys from different laboratories (and presumably different genetic pools) were begun.